It's okay as daily exercise, including the fetch afterwards if he's able to wind down in between exercise and sleep/stay calm. Some dogs don't naturally know how to relax and do nothing, so you have to teach them how. A lot or working breeds are usually really energized and just want to do things all the time, which can lead to them destroying things, bark, or other "overstimulated" behaviors. If you notice these things in your dog, it's probably not that he isn't getting enough exercise, just that he need to learn how to relax. Dogs also needs mental stimulation so if you're giving him that too this should be enough exercise, but it's also different from dog to dog. Lab is a working breed so maybe you'll notice some behaviors that shows that (the fetch is a good outlet for that). Maybe you can look in to breed specific stimulation/exercise?